SPOILER ALERT! This post contains information about key plot points in the season premiere of Mad Men. If you haven’t watched Sunday’s episode, continue at your own risk.

Oh Don, no! We should know not to expect too much from the tortured ad man by now, but we were hoping Megan had helped him change his ways.

The season 6 premiere of Mad Men ended with a twist that’s sure to rile up fans and have people asking questions. After a sunny vacation to Hawaii, Don fell back into old habits and into a new bed with a mysterious woman, Sylvia, played by Freaks and Geeks and ER actress Linda Cardellini. Don and Sylvia didn’t seem to be trying all that hard to keep their affair from their spouses, but Cardellini and showrunner Matthew Weiner have done all they can to make Sylvia’s arrival a surprise. Not even Cardellini’s best friends or immediate family knew about her participation in the show, so when we caught up with her before last night’s season opener, she apologized in advance for everything she couldn’t say. Still, Cardellini managed to tease her character’s upcoming arc a bit – and reminisce on the other time she “homewrecked” a TV power couple, as the girl who came between Cory and Topanga on Boy Meets World in 1998.

On the inevitable backlash to her character: “I think everybody handles it differently and everybody has their level of sensitivity to it. You know, the hard thing about audiences not liking what a character does is that they sometimes take it out on the actor personally and that’s something that you know when you become an actor or actress, but it’s always hard to deal with when it actually happens.”

On her co-stars: “I’d done a movie before with Vincent Kartheiser and I had done another movie recently with John Slattery, so I had acted with them before. … It’s a very, very good group of people that have been good to me. I wish I could tell you more stories, but then it would allude to who I hang out with on set. It would give away certain things that I probably can’t.”

On keeping her role a secret: “It’s really fun! Usually people know what you’re doing, especially with the Internet. People know what you’re doing sometimes even before you know. To have it be a surprise for my friends and family who haven’t seen me do anything for a while because I’ve been home with my daughter, it’ll be a really fun surprise.”

On filming a love scene with Jon Hamm (no comment on his alleged aversion to underwear): “You come to set and you know that that’s what you’re going to be doing, and it’s part of your job and it’s a wonderful part of the story. You do what you’re meant to do with the most kindness and generosity toward each other as you can and you hope that it turns out well. It’s such a great part of the episode.”

On Sylvia’s look: “That’s not my hair. You know, it takes people a second to realize that it’s me. I had several fittings, and the things underneath what you’re wearing, they have to be true to the period so you can look and feel the way that they did at the time, which helps. It informs your character and the way that your body moves and can’t move, in some ways.”

On the vibe on set: “It’s a very different vibe from something like Freaks and Geeks, where the scripts are incredible but it’s a lot looser – there was improv and it was also a period piece but it was a different period and there was more of a naturalistic, loose vibe to the work. When it comes to something like Mad Men, it’s very precise, and you may not even understand the part that you’re playing in the bigger picture. You do precisely what is instructed in terms of what is worded perfectly and what is in the stage directions. Some shows operate like that, and some shows don’t. All I can say is it’s very precise, and it’s so deeply structured. The show is so deeply structured so to veer from it or to think that you’re more an authority on it doesn’t make any sense because there are things that need to precisely happen that you don’t even understand in the moment.”

On filming moment-to-moment: “Matt never gives it away! The secretive part isn’t only for the audience. It’s for everybody around set. I think it keeps everybody on their toes and guessing and it keeps everybody involved in the future of the show. I think it’s interesting on one level because the question marks, if you let them get to you, can drive you crazy, but it doesn’t because you just sort of have to jump in the water and swim with everybody. It’s pretty fun. And it’s unique. There are not that many jobs as an actor where you don’t get to know what you’re character will be doing from episode to episode. It’s sort of a giant unknown and you go along and trust. I think that’s what’s really special about Mad Men.”

On being “the other woman” on a different show: “I didn’t realize what I was stepping into by breaking up Cory and Topanga. I hadn’t really watched that show. I was just so excited to have a job on television and I really didn’t realize how vehemently people would react to my breaking them up. People are still mad about it! Now that [Girl Meets World is happening] people are really happy for them. I think people have fears that Lauren will come back and haunt them, which would be terrible for their relationship and their poor child!”